What this MCB is and what the ratings mean for fit
The Siemens 5SJ4101-8HG40 is a 1-pole SENTRON miniature circuit breaker with a D tripping characteristic, rated 10 kA according to EN 60898 and 15 kA according to IEC 60947-2. The D curve means it tolerates high inrush currents — typically motor starting or transformer energization — without nuisance tripping, because the magnetic trip threshold is higher than on B or C curves. The 15 kA breaking capacity under IEC 60947-2 gives you headroom for industrial panels where fault current at the board exceeds the 10 kA residential standard. It snaps onto standard DIN rail and occupies one modular width (18 mm), so it fits a standard panel layout with no special adapter.